The present invention relates to an error correction encoding/decoding method and an apparatus therefor to be employed in radio transmission systems, such as digital mobile telephones, portable telephones and so forth.
In the conventional digital mobile telephone systems or portable telephone systems, a strong error correction encoding is employed in order to maintain a predetermined level of quality even under poor transmission conditions (e.g. high channel error ratio). One of such strong error correction codes is a convolutional code. As a decoding method for the convolutional code, there is viterbi decoding employing a trellis diagram. As disclosed in "Points of Error Correction Encoding Technology", Japan Industrial Technology Center Co., Ltd., published on Mar. 20, 1986, pages 45.about.48, or Japanese Unexamined Patent Publication (Kokai) No. 2-215232, viterbi decoding is a decoding method, in which one of a plurality of known code streams, having the closest code metric to the received code stream, is selected as a maximum likelihood path, and decoded output data corresponding to the selected path is obtained.
FIG. 1 is a schematic block diagram of an error correction encoding/decoding apparatus employing the conventional viterbi decoding method. For reception data a metric is calculated in a branch metric calculation circuit 21 with respect to each branch (branch line from a certain state to the next state in the trellis diagram) corresponding to each information bit. Then, the branch metric derived by the branch metric calculation circuit 21 is added to a metric of an accumulated path (a sequence of the branches corresponding to the information signal stream) up to the immediately preceding branch stored in a path metric storage circuit 23 by an ACS (Add-Compare-Select) circuit 22. After that, the metrics in each state are compared, and then the maximum likelihood path is selected and stored in a path memory 24 as a path select signal. Also, the value of a path metric is updated and stored in the path metric storage circuit 23. Furthermore, the path metric value is decoded by maximum likelihood decoding by a maximum likelihood decoding circuit 25 and then output as decoded data.
Even with such conventional error correction decoding apparatus as set forth above, bit errors caused in the transmission line can be corrected to a certain precision level by utilizing a redundancy added in convolutional encoding.
However, the conventional error correction encoding/decoding apparatus as set forth above is adapted to only use the redundancy added in the convolutional encoding without using the redundancy of the information source even in the case that the parameter having correlation between consecutive frames is included as a part of information bits to be transmitted so as to have a certain transition probability to define a value in a current frame relative to the value in the previous frame. Therefore, under poor transmission conditions, there is high possibility that correct error correction cannot be performed.